I would ask the form teacher exactly what happened and how it's being dealt with. And better in person (followed up by email if there are still points of concern. Or is it impossible for you to get there?
Even reliable DC can inadvertently give an incomplete account of events (or, as discipline of one child may not be readily visible to others, may not have the full picture) so best to start with enquiry.
ds has autism and does not communicate well. the class teacher left at easter. the incident happened before easter. following the after school sports club tonight, ds ws afaid to go to the toilet alone. he will not talk about it further.
it is a bit late to punish the other child really. I am more interested in preventing a further incident.
Oh dear. It sounds as if you do need to go in to talk to the new teacher about DC's needs in the round, mention the legacy of this incident, and discuss what is the best support to help him regain confidence in this specifically and in general.
Inform the new teacher what you have said here. Ask if DS has been going to toilet in school hours alone this term. Ask for reassurance and support for DS, to be accompanied if necessary or entrance/exit to toilets supervised when he is in there. Does he have 1-1 support? If so this level of supervision whoule be asy for them to provide.
imo it is too late to resolve the in itial incident, you need to deal with his fears now.
is there any chance he is getting picked on by other children in the class or do you think this was a one off incident that could have happended to anyone?